Conventionally, an optical signal transmitter and an optical signal transmission system employing a method for subjecting video signals, which have undergone frequency-division multiplexing, to frequency modulation as a single unit (this method will be hereinafter referred to as an “FM batch conversion method”) are known as an optical signal transmitter and an optical signal transmission system used for optical transmission of multichannel video signals that have undergone frequency-division multiplexing and that have undergone amplitude modulation or quadrature amplitude modulation.
An optical signal transmitter and an optical signal transmission system that employ this FM batch conversion method are disclosed in Non-patent Document 1.
FIG. 1 shows a structure of a conventional optical signal transmitter and a conventional optical signal transmission system that employ the FM batch conversion method. FIGS. 2A, 2B, and 2C show signal forms at point “A,” point “B,” and point “C” of FIG. 1, respectively. The optical signal transmission system of FIG. 1 is comprises an optical signal transmitter 80 including an FM batch conversion circuit 81, a light source 82, and an optical amplification circuit 83, an optical transmission path 85, an optical signal receiver 90 including a photoelectric conversion circuit 91 and an FM demodulation circuit 92, a set-top box 93, and a television receiver 94. Signal spectra at point “A,” point “B,” and point “C” of FIG. 1 are shown in FIGS. 2A, 2B, and 2C, respectively. The same applies to point “A,” point “B,” and point “C” of each figure shown below.
In the optical signal transmitter 80 of FIG. 1, frequency-multiplexed video signals shown in FIG. 2A are converted into one wideband frequency-modulated signal shown in FIG. 2B by the FM batch conversion circuit 81. The frequency-modulated signal is subjected to intensity modulation by the light source 82, and is further subjected to optical amplification by the optical amplification circuit 83, and is transmitted to the optical transmission path 85. In the optical signal receiver 90, the frequency-modulated signal that has undergone intensity modulation is photoelectrically converted by the photoelectric conversion circuit 91, and is returned to an electric signal. This electric signal, which is a wideband frequency-modulated signal, is subjected to frequency demodulation by the FM demodulation circuit 92, and the frequency-multiplexed video signals are demodulated as shown in FIG. 2C. The demodulated video signals pass through the set-top box 93, and reach the television receiver 94, whereby a desired video channel is selected.
FIG. 3 shows the structure of an FM batch conversion circuit that is applicable to the FM batch conversion method (see Patent Document 1, Non-patent Document 2, Non-patent Document 3, for example). The FM batch conversion circuit shown in FIG. 3 uses an optical frequency modulation portion and an optical frequency local oscillation portion. The FM batch conversion circuit 81 comprises the optical frequency modulation portion 71, the optical frequency local oscillation portion 72, an optical multiplexer 73, and a photodiode 74.
When frequency modulation is performed with a frequency fs by use of a carrier light source having an optical frequency fo in the optical frequency modulation portion 71 of the FM batch conversion circuit 81, an optical frequency Ffmld of an optical signal in the output of the optical frequency modulation portion 71 is expressed as in the following equation:Ffmld=fo+δf·sin(2π·fs·t)  (1)where δf is a frequency deviation. A DFB-LD (Distributed Feed-Back Laser Diode) is used as the carrier light source of the optical frequency modulation portion 71.
In the optical frequency local oscillation portion 72, oscillation is performed by use of an oscillation light source having an optical frequency f1. An optical signal transmitted from the local oscillation portion 72 and an optical signal transmitted from the optical frequency modulation portion 71 are multiplexed by the optical multiplexer 73. The DFB-LD is used as the oscillation light source of the optical frequency local oscillation portion 72. The two optical signals multiplexed by the optical multiplexer 73 are detected by the photodiode 74 that is an optical heterodyne detector. The frequency f of the electric signal detected thereby is expressed as follows:f=fo−f1+δf·sin(2π·fs·t)  (2)Herein, if the optical frequency of the carrier light source of the optical frequency modulation portion 71 and the optical frequency of the oscillation light source of the optical frequency local oscillation portion 72 are caused to come close to each other, it is possible to obtain an electric signal whose frequency is modulated to have an intermediate frequency fi=fo−f1 of several GHz and have a frequency deviation δf as shown in FIG. 2B.
Generally, the modulation by an input electric current allows the DFB-LD to have an optical frequency varied in the range of several GHz in accordance with the input electric current, and hence a value of several GHz can be obtained as the frequency deviation δf. For example, a multichannel AM video signal or QAM video signal that have undergone frequency multiplication so as to have a frequency range of about 90 MHz to about 750 MHz can be converted by the FM batch conversion circuit into a frequency-modulated signal having a frequency band of about 6 GHz in which the intermediate frequency fi=fo−f1 becomes equal to about 3 GHz as shown in FIG. 2B.
FIG. 4 shows the structure of an FM demodulation circuit applicable to the optical signal receiver 90. The FM demodulation circuit 92 shown in FIG. 4 is an FM demodulation circuit by delay-line detection, and comprises a limiter amplifier 76, a delay line 77, an AND gate 78, and a low-pass filter 79.
In the FM demodulation circuit 92, a frequency-modulated optical signal that has been input is shaped into a square wave by the limiter amplifier 76. The output of the limiter amplifier 76 is branched into two output parts, one of which is input to an input terminal of the AND gate 78 and the other of which undergoes a polarity reversal, is then delayed by time t by means of the delay line 77, and is input to an input terminal of the AND gate 78. The output of the AND gate 78 is smoothed by the low-pass filter 79, and is turned into frequency-demodulated output (see Non-patent Document 1, for example).
A double-tuned frequency discriminator having a resonance circuit, a Foster-Seeley frequency discriminator, and a ratio detection type FM demodulator can be mentioned as a circuit form of the FM demodulation circuit, in addition to the FM demodulation circuit by delay-line detection described here.
Patent Document 1: Japanese Patent No. 2700622;
Non-patent Document 1: international standard, ITU-T J. 185, “Transmission equipment for transferring multi-channel television signals over optical access networks by FM conversion;”
Non-patent Document 2: Shibata et al. “Optical image distribution system using an FM batch conversion method,” Institute of Electronics, Information and Communication Engineers, Technical Journal B, Vol. J83-B, No. 7, July, 2000, pp. 948-959;
Non-patent Document 3: Suzuki et al. “Pulsed FM batch conversion modulation analog optical CATV distribution method” Institute of Electronics, Information and Communication Engineers, Autumn Conference, B-603, 1991.